Some wireless communication networks are proprietary while others are deployed in conformity with one or more standards and accommodate equipment manufactured by various vendors. One such standards-based network is the Universal Mobile Telecommunications System (UMTS) standardized by the Third Generation Partnership Project (3GPP), which is a collaboration of groups of telecommunications associations that generate globally applicable mobile phone system specifications within the scope of the International Mobile Telecommunications-2000 project of the International Telecommunication Union (ITU). Efforts are currently underway to develop an evolved UMTS standard, which is typically referred to as UMTS Long Term Evolution (LTE) or Evolved UMTS Terrestrial Radio Access (E-UTRA).
According to Release 8 of the E-UTRA or LTE standard or specification, downlink communications from a base station (referred to as an “enhanced Node-B” or simply “eNB”) to a wireless communication device (referred to as “user equipment” or “UE”) utilize orthogonal frequency division multiplexing (OFDM). In OFDM, orthogonal subcarriers are modulated with a digital stream, which may include data, control information, or other information, to form a set of OFDM symbols. The subcarriers may be contiguous or non-contiguous and the downlink data modulation may be performed using quadrature phase shift-keying (QPSK), 16-ary quadrature amplitude modulation (16QAM) or 64QAM. The OFDM symbols are configured into a downlink sub frame for transmission from the base station. Each OFDM symbol has a time duration and is associated with a cyclic prefix (CP). A cyclic prefix is essentially a guard period between successive OFDM symbols in a sub frame. According to the E-UTRA specification, a normal cyclic prefix is about five (5) microseconds and an extended cyclic prefix is about 16.67 microseconds. Data from the serving base station is transmitted on physical downlink shared channel (PDSCH) and control information is signaled on a physical downlink control channel (PDCCH).
In contrast to the downlink, uplink communications from the UE to the eNB utilize single-carrier frequency division multiple access (SC-FDMA) according to the E-UTRA standard. In SC-FDMA, block transmission of QAM data symbols is performed by a first discrete Fourier transform (DFT)-spreading (or precoding) followed by subcarrier mapping to a conventional OFDM modulator. The use of DFT precoding allows a moderate cubic metric/peak-to-average power ratio (PAPR) leading to reduced cost, size and power consumption of the UE power amplifier. In accordance with SC-FDMA, each subcarrier used for uplink transmission includes information for all the transmitted modulated signals, with the input data stream being spread over them. The data transmission in the uplink is controlled by the eNB, involving transmission of scheduling grants (and scheduling information) sent via downlink control channels. Scheduling grants for uplink transmissions are provided by the eNB on the downlink and include, among other things, a resource allocation (e.g., a resource block size per one millisecond (ms) interval) and an identification of the modulation to be used for the uplink transmissions. With the addition of higher-order modulation and adaptive modulation and coding (AMC), large spectral efficiency is possible by scheduling users with favorable channel conditions. The UE transmits data on the physical uplink shared channel (PUSCH). The physical control information is transmitted by the UE on the physical uplink control channel (PUCCH).
E-UTRA systems also facilitate the use of multiple input and multiple output (MIMO) antenna systems on the downlink to increase capacity. As is known, MIMO antenna systems are employed at the eNB through use of multiple transmit antennas and at the UE through use of multiple receive antennas. A UE may rely on a pilot or reference symbol (RS) sent from the eNB for channel estimation, subsequent data demodulation, and link quality measurement for reporting. The link quality measurements for feedback may include such spatial parameters as rank indicator or the number of data streams sent on the same resources, precoding matrix index (PMI), and coding parameters, such as a modulation and coding scheme (MCS) or a channel quality indicator (CQI). For example, if a UE determines that the link can support a rank greater than one, it may report multiple CQI values (e.g., two CQI values when rank=2). Further, the link quality measurements may be reported on a periodic or aperiodic basis, as instructed by an eNB, in one of the supported feedback modes. The reports may include wideband or subband frequency selective information of the parameters. The eNB may use the rank information, the CQI, and other parameters, such as uplink quality information, to serve the UE on the uplink and downlink channels.
In the context of the Release-8 specification of Long Term Evolution (LTE) system developed by third generation partnership project (3GPP) that is based on Orthogonal Frequency Division Multiplexing (OFDM) for downlink transmissions, the eNB-to-UE link consists of typically 1-3 OFDM symbols (length is signaled via the physical control format indicator channel (PCFICH)) at the beginning of each 1-ms sub-frame for control channel, i.e., PDCCH, transmissions. Typically an OFDM symbol comprises of an integer number of time units (or samples), where a time unit denotes a fundamental reference time duration. For example, in LTE, the time unit corresponds to 1/(15000×2048) seconds. Thus, the PDCCH transmissions are a first control region with a fixed starting location (contemporaneously) at the first OFDM symbol in a sub-frame. All the remaining symbols in a sub-frame after the PDCCH are typically for data-carrying traffic, i.e., PDSCH, assigned in multiples of Resource Blocks (RBs). Typically, an RB comprises of a set of subcarriers and a set of OFDM symbols. The smallest resource unit for transmissions is denoted a resource element which is given by the smallest time-frequency resource unit (one subcarrier by one OFDM symbol). For example, an RB may contain 12 subcarriers (with a subcarrier separation of 15 kHz) with 14 OFDM symbols with some subcarriers being assigned as pilot symbols, etc. Typically, the 1 ms sub-frame is divided into two slots, each of 0.5 ms. The RB is sometimes defined in terms of one or ore more slots rather than sub-frames. According to the Release-8 specification, the uplink communication between the UE and eNB is based on Single-Carrier Frequency Division Multiple Access (SC-FDMA), which is also referred to as Discrete Fourier Transform (DFT)-spread OFDM. It is also possible to have non-contiguous uplink allocations by send uplink control information and uplink data on non-contiguous subcarriers. A virtual resource block is a resource block whose subcarriers are distributed (i.e., non-contiguous) in frequency, whereas a localized RB is an RB whose subcarriers are contiguous in frequency. Virtual RB may have improved performance due to frequency diversity. Release-8 UEs typically share resources in the frequency domain (i.e., on an RB-level or in multiples of an RB) rather than in time in any individual sub-frame on the downlink.
The PDCCH contains control information about the downlink control information (DCI) formats or scheduling messages, which inform the UE of the modulation and coding scheme, transport block size and location, precoding information, hybrid-ARQ information, UE Identifier, etc. that is required to decode the downlink data transmissions. This control information is protected by channel coding (typically, a cyclic-redundancy check (CRC) code for error detection and convolutional encoding for error correction) and the resulting encoded bits are mapped on the time-frequency resources. For example, in LTE Rel-8, these time-frequency resources occupy the first several OFDM symbols in a sub-frame. A group of four Resource Elements is termed as a Resource Element Group (REG). Nine REGs comprise a Control Channel Element (CCE). The encoded bits are typically mapped onto either 1 CCE, 2 CCEs, 4 CCEs or 8 CCEs. These four are typically referred to as aggregation levels 1, 2, 4 and 8. The UE searches the different hypotheses (i.e., hypotheses on the aggregation level, DCI Format size, etc) by attempting to decode the transmission based on allowable configurations. This processing is referred to as blind decoding. To limit the number of configurations required for blind decoding, the number of hypotheses is limited. For example, the UE does blind decoding using the starting CCE locations as those allowed for the particular UE. This is done by the so-called UE-specific search space, which is a search space defined for the particular UE (typically configured during initial setup of a radio link and also modified using RRC message). Similarly a common search space is also defined that is valid for all UEs and might be used to schedule broadcast downlink information like Paging, or Random access response, or other purposes.
The control messages are typically encoded using convolutional encoders. The control region includes a Physical Hybrid ARQ Indicator channel or the PHICH that is used to transmit hybrid ARQ acknowledgments.
Each communication device searches the control region in each subframe for control channels (PDCCHs) with different downlink control indicator (DCI) formats using blind detection, where the PDCCH CRC is scrambled with either a communication device's C-RNTI (UEID) if it is for scheduling data on the Physical Downlink Shared Channel (PDSCH) or Physical Uplink Shared Channel (PUSCH) or scrambled with SI-RNTI, P-RNTI, or RA-RNTI if it is for scheduling broadcast control (system information, paging, or random access response respectively). Other scrambling types include joint power control, semi-persistent scheduling (SPS), and a temporary C-RNTI for use with scheduling some random access messages.
A particular user equipment must locate the control channel elements corresponding to each PDCCH candidate it is to monitor (blindly decode for each subframe control region). The CRC of each PDCCH will be masked by a unique identifier corresponding to the user equipment that the base unit is trying to schedule. The unique identifier is assigned to the UE by its serving base unit. This identifier is known as a radio network temporary identifier (RNTI) and the one normally assigned to each UE at call admission is the cell RNTI or C-RNTI. A UE may also be assigned a semi-persistent-scheduling C-RNTI (SPS C-RNTI) or a temporary C-RNTI (TC-RNTI). When a UE decodes a PDCCH it must apply its C-RNTI in the form of a mask to the PDCCH CRC for successful PDCCH decoding to occur. When a UE successfully decodes a PDCCH of a particular DCI format type it will use the control information from the decoded PDCCH to determine, for example, the resource allocation, Hybrid ARQ information, and power control information for the corresponding scheduled downlink or uplink data transmission. The legacy DCI format type 0 is used for scheduling uplink data transmissions on the Physical Uplink Shared Channel (PUSCH) and DCI format type 1A is used for scheduling downlink data transmissions on the Physical Downlink Shared Channel (PDSCH). Other DCI format types are also used for scheduling PDSCH transmissions including DCI format 1, 1B, 1D, 2, 2A each corresponding to a different transmission mode (e.g., single antenna transmissions, single user open loop MIMO, multi-user MIMO, single user close loop MIMO, rank-1 precoding). Also there are legacy DCI format 3 and 3A for scheduling the transmission of joint power control information. PDCCH DCI format 0, 1A, 3, and 3A all have the same size payload and hence the same coding rate. So only one blind decoding is required for all of 0, 1A, 3, 3A per PDCCH candidate. The CRC is then masked with C-RNTI to determine if the PDCCH was DCI format type 0 or 1A and a different RNTI if it is 3 or 3A. DCI format type 0 and 1A are distinguished by DCI type bit in the PDCCH payload itself (i.e. part of the control information on one of the control information fields). A UE is always required to search for all of DCI formats 0, 1A at each PDCCH candidate location in the UE specific search spaces. There are four UE specific search spaces for aggregation levels 1, 2, 4 and 8. Only one of the DCI format types 1, 1B, 1D, 2, or 2A is assigned at a time to a UE such that a UE only needs to do one additional blind decoding per PDCCH candidate location in the UE specific search space besides the one blind decoding needed for the 0, 1A DCI types. The PDCCH candidate locations are the same for the DCI format types when they are located in the UE specific search spaces. There are also two 16 CCE common search spaces of aggregation level 4 and 8 respectively that are logically and sometimes physically (when there are 32 or more control channel elements) adjacent to the UE specific search spaces. In the common search spaces a UE monitors DCI types 0, 1A, 3, and 3A as well as DCI format type 1C. DCI format type 1C is used for scheduling broadcast control which includes paging, random access response, and system information block transmissions. DCI 1A may also be used for broadcast control in the common search spaces. DCI 0 and 1A are also used for scheduling PUSCH and PDSCH in common search spaces. A UE is required to perform up to 4 blind decodings in the L=4 common search space and 2 blind decodings in the L=8 common search space for DCI formats 0, 1A, 3, and 3A and the same number again for DCI 1C since DCI 1C is not the same size as DCI 0, 1A, 3 and 3A. A UE is required to perform (6, 6, 2, 2) blind decodings for L=(1, 2, 4, 8) UE specific search spaces respectively where L refers to the aggregation level of the search space. The total maximum number of blind decoding attempts a UE is then require to perform per subframe control region is therefore 44 (=2×(6, 6, 2, 2)+2×(4, 2)). A hashing function is used by the serving base unit and the UE to find the PDCCH candidate locations in each search space. The hashing function is based on the UE C-RNTI (or sometimes the TC-RNTI), aggregation level (L), the total number of CCEs available in the control region (Ncce), the subframe number or index, and the maximum number of PDCCH candidates for the search space.
Home-base stations or femto-cells are referred to as Home-eNBs (HeNBs) in the present disclosure. A HeNB can either belong to a closed subscriber group (CSG) or can be an open-access cell. A CSG is set of one or more cells that allow access to only certain a group of subscribers. HeNB deployments where at least a part of the deployed bandwidth (BW) is shared with macro-cells are considered to be high-risk scenarios from an interference point-of-view. When UEs connected to a macro-cell roam close to a HeNB, the uplink of the HeNB can be severely interfered with particularly when the HeNB is far away (for example >400 m) from the macro-cell, thereby, degrading the quality of service of UEs connected to the HeNB. Currently, the existing Rel-8 UE measurement framework can be made use of identify the situation when this interference might occur and the network can handover the UE to an inter-frequency carrier which is not shared between macro-cells and HeNBs to mitigate this problem. However, there might not be any such carriers available in certain networks to handover the UE to. Further, as the penetration of HeNBs increases, being able to efficiently operate HeNBs on the entire available spectrum might be desirable from a cost perspective.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.